In today's high speed communication systems, optical components are employed in order to transmit information using optical signals. Usually, optical signals are transmitted over optical fibers, which, unfortunately, distort the transmitted signal due to different transmission channel characteristics at different wavelengths. The distortion may comprise wavelength-specific attenuation or chromatic dispersion, the later resulting when signals components at different wavelengths propagate with different velocities along the optical communication channel.
In order to compensate for the distortion, a digital filter may be employed at the receiver to improve the signal's quality for a subsequent detection of the transmitted information. For digitally filtering the received optical signal, first an optical coherent demodulation and, subsequently, an optical-to-electrical conversion upon the basis of e.g. light sensitive diodes are performed. Nevertheless, the resulting digital signal still comprises residual distortion, e.g. chromatic dispersion, which can be reduced by way of digital filtering.
FIG. 7 shows a structure of a conventional chromatic dispersion filter as known from M. Kuschnerov, F. N. Hauske, K. Piyawanno, B. Spinnler, A. Napoli, and B. Lankl, “Adaptive Chromatic Dispersion Equalization for Non-Dispersion Managed Coherent Systems”, OFC 2009, paper OMT1. The filter structure comprises a Fast Fourier Transformer (FFT) 701 for transforming a time domain signal into frequency domain. The resulting frequency domain signal multiplied by a multiplier 703 with a filter coefficient, wherein the multiplier 703 has an output connected to an inverse Fast Fourier Transformer (IFFT) 705 for transforming the multiplied signal into time domain. The time domain signal is then provided via a feedback loop to a subtractor 707 subtracting an expectation power from the time domain signal. The resulting signal is provided to a further Fast Fourier Transformer 709 transforming the resulting signal into frequency domain, which signal is subsequently multiplied by a further multiplier 711 with the original frequency domain signal provided by the FFT 701. The output of the multiplier 711 is provided to an adder 713 having a feedback loop for adding a previous addition result to the output signal of the multiplier to obtain a channel coefficient provided to the multiplier 703. As depicted in FIG. 7, time domain signals and frequency domain signals are exploited in order to update the filter coefficients.
With reference to FIG. 7, from a complex value time domain signal s(t), an error criterion u(t)=|s(t)|2−R is derived, where R denotes the above mentioned expectation power. This approach relates to the known constant modulus algorithm (CMA). In order to adapt the filter in the frequency domain, the error signal u(t) has to be transferred to the frequency domain by the FFT 701 in order to update the filtering function, i.e. the filter coefficients. After a plurality of consecutive updates, the filter will approximate the ideal filter function Hdis−1(ω) determining the filter coefficients and representing an inverse of the channel filter function introducing chromatic dispersion.